This invention relates generally to electrophotographic printing, and more particularly concerns an improved optical system for projecting a light image onto a moving photoconductive surface having illumination control therefor.
In the process of electrophotographic printing, for example, as disclosed in U.S. Pat. No. 2,297,691, issued to Carlson in 1942, an imaging bearing member or photoconductive member is charged to a substantially uniform potential in order to sensitize its surface. Thereafter, the charged photoconductive surface is exposed to a light image of an original document. As a consequence of the exposure, the charge is selectively dissipated in the irradiated areas in accordance with the light intensity projected onto the charged photoconductive surface recording an electrostatic latent image thereon. Development of the electrostatic latent image recorded on the photoconductive surface is achieved by bringing a developer mix into contact therewith. Typical developer mixes generally comprise dyed or colored thermoplastic particles, known in the art as toner particles, which are mixed with coarser carrier beads, such as ferromagnetic granules. The developer mix is selected such that the toner particles acquire the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface. The toner powder image is thereupon transferred to a sheet of support material. After the toner powder image has been transferred to the sheet of support material it is permanently affixed thereto.
In order to insure that a copy of the original document is of high quality, a uniformly high level of illumination is required for the system. To this end, the quality of the copy is dependent upon the characteristics of the exposure system. The exposure system is adapted to regulate the intensity of the light image. The intensity of the light image transmitted to the charged photoconductive surface determines the level to which the photoconductive surface is discharged. This, in turn, is instrumental in defining copy contrast. For example, if the exposure system only partially discharges the photoconductive surface, the charge remaining thereon will be higher than normal and the relative difference between the charge on the photoconductive surface and that of the development system will be small. Under these circumstances, a small amount of toner particles will be deposited on the electrostatic latent image resulting in a gray copy. Contrawise, if the intensity of the light image impinging the photoconductive surface is too great, the charge on the photoconductive surface will be dissipated to an appreciably greater level resulting in a toner powder image having high toner powder density. A copy resulting from this powder image will be extremely black and may have high background.
The intensity of illumination may be adjusted by controlling the exposure time. In an electrophotographic printing machine employing a rotating photoconductive drum and an optical system moving in synchronism therewith to scan a stationary original document, the area of the exposure slit determines the exposure time. When the intensity of the light image is low, a more complete discharge of the photoconductive surface is obtained by increasing the exposure time. Contrawise, if the light image intensity is high, the exposure time may be reduced. Inasmuch as the photoconductive drum and optical system move in synchronism with one another, the light image, relative to the photoconductive surface, is stationary. This permits the exposure slit to control the exposure time or duration within broad limits to compensate for projected light image intensity variations.
Various types of prior art devices have been developed to achieve the foregoing. U.S. Pat. No. 3,049,968 issued to Johanson in 1962, discloses an exposure system employing a manually operable slit aperature. This teaches the use of masks to manually vary the size of an exposure slit. Bauer, U.S. Pat. No. 3,672,759 issued in 1972, describes the use of a motor adapted to vary an exposure slit width in accordance with the intensity of the light transmitted to a photoconductive drum. An additional patent relating thereto is U.S. Pat. No. 3,438,704 issued to Schoen in 1969. This patent teaches the use of a variable width exposure slit. The exposure slit width is adjusted in accordance with a measured light intensity signal. The light intensity signal is derived from a wheatstone bridge arrangement and the width of the slit is varied accordingly.
However, none of the prior art devices appear to provide a simple adjustment for controlling exposure time as a function of the characteristics of the original document being reproduced.
Accordingly, it is an object of the present invention to improve the illumination system by providing a control therefor which is readily adjustable to regulate the duration of time a light image is projected onto a charged photoconductive surface.